The present invention relates to a novel oxygen enriching membrane which exhibits the desired oxygen selectivity and permeability under a low pressure difference characterized in that it is prepared from siloxylated cellulose and/or siloxylated glass.
The air we breathe consists of about 20.9% of oxygen and about 78.1% of nitrogen. We call the membrane through which oxygen passes more selectively than the other gases as oxygen enriching membrane. Oxygen enriched air produced by the membrane elevates the efficiency of internal-combustion engine of car, which results in saving of energy, and further it may be widely used in the area of medical care, oxidizing procedure and environmental engineering. In addition, enriched nitrogen obtained as a side product during the oxygen enriching process can be utilized efficiently in various industrial areas which require the nitrogen enriched condition.
The oxygen enriching membrane enables us to separate gas by a simple device, and thus we can use it conveniently without any restriction on the space or portability. Accordingly, the gas separation technique using a membrane has been highly improved since 1980. The membrane-separation method is rather inadequate for obtaining a large amount of oxygen in a high density than the cryogenic process. Recently, however, the membrane-separation method becomes the center of interest because it requires less energy for the separation.
Since Kammermeyer first applied silicone rubber in gas separation on 1957(see, K. Kammermeyer, Ind. Eng. Chem., 49, 1685, 1957), it has been recognized that methyl-based polysiloxane has the best oxygen permeability than any kind of polymers until the appearance of polytrimethylsilylpropyne(see, R. L. Riley et al., U.S. Pat. No. 4,243,701). However, since the membrane can be more easily torn as it becomes thinner for the purpose of improving permeability, further researches were required. As a result of the researches, the oxygen enriching membrane which was made from a copolymer of siloxane and carbonate was identified to be strong, and thus, was commercialized first(see, W. J. Ward III, J. Membrane Sci., 1, 99, 1976). Then, several methods, such as for example, a method of using a graft copolymer of styrene and dimethylsiloxane, a method of introducing amide groups into the main chain structure by using tetramethyldisiloxane and aliphatic dicarboxylic acid, a method of substituting various siloxane groups for para-position of polystyrene, etc. were approached from this point of view(see, Y. Kawakami, J. Polym. Sci., Part A, 25, 1591, 1987).
Derivatives substituted by fluorine were also used as the material for preparing an oxygen enriching membrane because solubility of oxygen gas into the membrane becomes higher as the fluorine content increases. For example, Yoshio et al. prepared an oxygen enriching membrane having the oxygen selectivity against nitrogen of 2.1 from the synthetic poly(phenylacetylene) derivative in which trifluoromethyl group is attached to benzene ring(see, Y. Hayakawa, M. Nishida, T. Aoki, and H. Muramnatsu, J. Polym. Sci., Part A, 30, 873, 1992), and Paul et al. prepared oxygen enriching membranes having oxygen selectivities against nitrogen of 6.37 and 7.47, respectively, from the synthetic tetra-halogenated polycarbonate produced by halogenating Bis-phenol A with chlorine, bromine, etc(see, D. R. Paul, J. Membrane Sci., 34, 185, 1987).
However, in spite of the constant efforts as mentioned above, the existing techniques still have the problems that oxygen permeability is apt to decrease as oxygen selectivity against nitrogen increases and that the energy required to separate oxygen increases as the pressure difference which is the driving force of gas separation through membrane increases for a good permeability.
Thus, the present inventors have extensively studied to develop an oxygen enriching membrane which has a good strength and also has an excellent oxygen enriching ability under a low pressure difference. As a result, we have found that such a purpose can be splendidly accomplished by using siloxylated cellulose or siloxylated glass as the material for oxygen enriching membrane, and then completed the present invention.
Therefore, it is an object of the present invention to provide a novel oxygen enriching membrane which has an outstanding oxygen selectivity and permeability under a low pressure difference, and which is prepared from siloxylated cellulose and/or siloxylated glass.